[Bug 260200] AHCI emulation throws CAM errors when backed by -t malloc memory disk devices

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260200

Aleksandr Fedorov <afedorov@FreeBSD.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |afedorov@FreeBSD.org,
                   |                            |markj@FreeBSD.org

--- Comment #2 from Aleksandr Fedorov <afedorov@FreeBSD.org> ---
This is because md(4) + memory backend doesn't support the DIOCGFLUSH ioctl
(BIO_FLUSH command):
https://github.com/freebsd/freebsd-src/blob/main/sys/dev/md/md.c#L650

pci_ahci.c:
https://github.com/freebsd/freebsd-src/blob/main/usr.sbin/bhyve/pci_ahci.c#L783

So I see two ways to solve this issue.

First, fix bhyve with patch like this:

diff --git a/usr.sbin/bhyve/block_if.c b/usr.sbin/bhyve/block_if.c
index 98c0f9f5f38b..3fb81bb86ae2 100644
--- a/usr.sbin/bhyve/block_if.c
+++ b/usr.sbin/bhyve/block_if.c
@@ -102,6 +102,7 @@ struct blockif_ctxt {
        int                     bc_ischr;
        int                     bc_isgeom;
        int                     bc_candelete;
+       int                     bc_canflush;
        int                     bc_rdonly;
        off_t                   bc_size;
        int                     bc_sectsz;
@@ -224,6 +225,8 @@ static int
 blockif_flush_bc(struct blockif_ctxt *bc)
 {
        if (bc->bc_ischr) {
+               if (bc->bc_canflush == 0)
+                       return (0);
                if (ioctl(bc->bc_fd, DIOCGFLUSH))
                        return (errno);
        } else if (fsync(bc->bc_fd))
@@ -463,7 +466,7 @@ blockif_open(nvlist_t *nvl, const char *ident)
        struct diocgattr_arg arg;
        off_t size, psectsz, psectoff;
        int extra, fd, i, sectsz;
-       int ro, candelete, geom, ssopt, pssopt;
+       int ro, candelete, canflush, geom, ssopt, pssopt;
        int nodelete;

 #ifndef WITHOUT_CAPSICUM
@@ -566,6 +569,12 @@ blockif_open(nvlist_t *nvl, const char *ident)
                        candelete = arg.value.i;
                if (ioctl(fd, DIOCGPROVIDERNAME, name) == 0)
                        geom = 1;
+               if (ioctl(fd, DIOCGFLUSH) == 0)
+                       canflush = 1;
+               else if (errno == ENOTSUP) {
+                       canflush = 0;
+                       errno = 0;
+               }
        } else
                psectsz = sbuf.st_blksize;

@@ -614,6 +623,7 @@ blockif_open(nvlist_t *nvl, const char *ident)
        bc->bc_ischr = S_ISCHR(sbuf.st_mode);
        bc->bc_isgeom = geom;
        bc->bc_candelete = candelete;
+       bc->bc_canflush = canflush;
        bc->bc_rdonly = ro;
        bc->bc_size = size;
        bc->bc_sectsz = sectsz;

Second, add a NOP support of the BIO_FLUSH cmd to md(4) (mdstart_malloc()).

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